The invention relates to a mass flowmeter of the thermal type.
Various mass flowmeters operating by the thermal principle are known.
When a gas or liquid flow whose mass flowrate is to be measured passes as a laminar or turbulent flow through a flow tube, a boundary layer is formed along the inner wall of the flow tube. Local heating of the tube, for example by means of a resistance wire coiled around the tube, causes heat to be transmitted to the gas or liquid flow by heat conduction through the tube wall and the boundary layer. The effect of the supplied heat is determined. The thermal conductance of the tube is determined as a variable value that is dependent on the composition of the material and the quantity of heat transmitted through its surface, and the thermal conductance of the gas or liquid boundary layer is determined as a value related to the specific heat capacity in the fluid to be measured.
The heat conductance of the flow tube is thus calculated from data on its constituent material, tube dimensions, etc., on the one hand, while on the other hand the thermal conductance of the fluid is calculated from its density and flowrate. The flowmeter of conventional type may be used as a simple mass flowmeter capable of determining the mass flowrate of a fluid of a known type on the basis of the above operating principle without interfering with the flow of the fluid.
A known mass flowmeter of the thermal type is described, for example, in EP no. 01201156.5. This known mass flowmeter comprises a heat-conducting flow tube provided with an upstream and a downstream temperature sensor composed of resistance wire coiled around the tube, and a control circuit for keeping the temperature difference between the sensors constant, the mass flowrate of the fluid flowing through the tube being determined from data of the control circuit.
Disadvantages of such a measuring system with windings provided around a flow tube are that it is not suitable for measuring the flow highly locally, cannot measure very low flowrates, has a comparatively slow response, and cannot be used in small spaces.